I am not sure if I have asked this before, and if so, my apologies.
I have the individual panes from the 1969 Prime Ministers series. Under UV light, the SG449a pane (G. H. Ried) appears differently from the other, having a somewhat orange hue. When viewed from the back, there is no discernible difference. ACSC indicates that this issue was printed as standard on Helecon paper but of the four panes, SG 446a, 447a and 449a were also printed on non-Helecon paper.
Am I lucky enough to have found a non-Helecon pane? I am not exactly sure what I should be looking for under UV light, front and back.
The difference appears more dramatic in real life than in the image of the front.